The present invention relates generally to a method of MR imaging and, more particularly, to a method and apparatus of split-blade data collection for PROPELLER MRI.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Fast Spin Echo (FSE) imaging is an imaging technique commonly used as an efficient method of collecting MRI data with minimal artifact. Generally, FSE requires that the refocusing B1 pulses be applied between each echo such that their phase is substantially identical to that of the initial spin phase after excitation, commonly referred to as the “CPMG” condition. If this condition is not met, the resulting MR signal is general highly sensitive to the strength of B1, and therefore will generally decay rapidly in successive echoes.
As a result, FSE imaging with diffusion weighting (FSE-DWI) may be difficult, in general, since even minute patient motion during application of diffusion weighting gradients leaves the spins with a spatially varying and unknown starting phase prior to the spin-echo train. A number of imaging techniques have been developed that alters the phase of the refocusing pulses to attempt to delay the inevitable signal decay. However, these known techniques have been shown to prolong the signal magnitude, but, in general, cause a spatially varying phase which alternates between successive echoes, i.e. the signal in odd echoes encode an additive phase α(x,y), and even echoes encode the opposite phase −α(x,y). This makes combining the two sets of echoes difficult.
FSE imaging is an imaging technique that has been implemented with a number of pulse sequence designs. For example, one FSE technique, which is commonly referred to as Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) imaging, encodes an MR signal by collecting data during an echo train such that a rectangular strip, or “blade”, through the center of k-space is measured. This strip is incrementally rotated in k-space about the origin in subsequent echo trains, thereby allowing adequate measurement of the necessary regions of k-space for a desired resolution.
Referring now to FIG. 5, a portion of a conventional pulse sequence 108 to acquire MR data in accordance with a PROPELLER protocol is shown. It should be noted that the phase encoding pulses, balancing gradients, and gradient crushers are not shown. The pulse sequence 108, in the illustrated example, is designed to acquire 12 spin-echoes 110 from a region of interest. The spin-echoes are all collected relative to single axis, e.g. Gx. In this regard, the 12 spin-echoes include odd spin-echoes as well as even spin-echoes. Each spin-echo 110 is acquired following an RF refocussing pulse 112 and during a frequency encoding pulse 114 a series of which are played out during steady-state conditions. The spin echo data is used to fill k-space which is schematically represented in FIG. 6.
FIG. 6 illustrates schematically a k-space 116 to be filled with MR data. With a conventional PROPELLER protocol, each echo acquired corresponds to a single line 118 of k-space 116. As such, for a 12 spin-echo data acquisition, each blade 120 of k-space includes 12 lines of data: In the illustrated example, each dashed line 122 represents an odd spin-echo trajectory and the solid lines 124 represent the even spin-echo trajectories. As shown, the even spin-echoes 124 are placed in a center of the k-space blade 120 and the odd spin-echoes 122 are placed about a periphery of the even spin-echo lines 124.
As is well-known, PROPELLER based imaging implements a rotation of the blades of k-space data with each echo-train. In this regard, the blade of k-space will be incrementally rotated about the center 126 of k-space with each echo-train acquisition until k-space is filled. When the k-space is filled, the MR data will undergo at least one of a number of known transformation techniques to generate an imaging space used to reconstruct an image of the subject. As discussed below, this protocol and processing is sufficient when a combination transmit/receive coil is used; however, is problematic when the MR study calls for separate transmit and receive coils.
It has been shown that the phase difference between the transmitter and receiver coil of the MR system to acquire the data is the same (e.g. with a T/R coil), odd and even lines of k-space can be combined into a single blade despite the presence of the +/−α(x,y) phase referenced above. Customarily, a Fourier process is implemented that exploits the conjugate symmetry between the odd and even lines of k-space. However, if data are collected using a separate transmit and receive coil, such that the relative phase difference between the refocusing pulses and the receiver is spatially varying, the Fourier process is unworkable. Thus, PROPELLER FSE-DWI as well as other similar constructed pulse sequences based on FSE imaging techniques have thus far been limited to use with a single transmit-receive coil. Further, it has been shown insufficient to simply use separate receive coils because artifacts will appear in the reconstructed image wherein the severity of the artifact will be a function of the relative change in phase between the transmit and receive coils.
It would therefore be desirable to have a system and method of MR imaging implementing a PROPELLER or similar imaging protocol with a separate transmit and receive coils for data acquisition with reduced or minimal image artifact.